Water-soluble NaGdF4∶Eu fluorescent nanoparticles modified by citrate were synthesized by hydrothermal method with stable fluorescent properties. It was found that the fluorescence of the solution of as-prepared particles could be quenched by Cu2+, and thus a new mathod to determine trace Cu2+ using NaGdF4∶Eu as fluorescent probe was established. A pH 10.0 and the concentration 1.0×10－3 mol·L-1 of NaGdF4∶Eu were selected for measurement. Besides, the effect of some foreign ions on the fluorescence signals was investigated and the interference of Fe3+ was found, which was eliminated by adding triethanolamine. The regression equation of standard curve was I=532－0.685c with the correlation coefficient of －0.998 4 when the concentration of Cu2+ was in the range of 3.33×10－6-1.33×10－4 mol·L-1, and the detection limit of 8.9×10－7 mol·L－1 and a RSD of 0.62% for 11 replicates of a 6.0×10－5 mol·L-1 Cu2+ solution were obtained, which suggest a wide linear analytical range, high sensitivity and high precision. Analytical applicability of the particles was demonstrated by tea sample analysis and the results of Cu2+ determination were in good agreement with those obtained by atomic absorption spectrometry. The reason for fluorescence quenching by Cu2+ can be explained in terms of combination of Cu2+ with citrate on the surface of NaGdF4∶Eu particles leading to a change in surface structure and the composition.